At present, liquid crystal displays as an example of liquid crystal display apparatuses have widely been popularized as various monitors for a personal computer and the like and as a display device for a cellular phone, and so on. In the future, the liquid crystal displays are expected to follow popularization more and more, such as development in use for big-screen television.
As a color display method for the liquid crystal displays, one called a micro-color filter method has been used widely.
The micro-color filter method effects full-color display by dividing one pixel into at least three sub-pixels and providing the three sub-pixels with color filters of three primary colors of red (R), green (G), and blue (B), respectively, thus having an advantage of readily realizing a high color-reproducing performance.
However, as a disadvantage of the micro-color filter method, a transmittance is ⅓ of a monochromatic display method, so that a light utilization efficiency is low.
This low light utilization efficiency leads to a high power consumption since it is necessary to increase a luminance of a back light or a front light when bright display is intended to be effected in a transmission-type liquid crystal display apparatus having the back light, a transflective (semi-transmission)-type liquid crystal display apparatus having the back light, or a reflection-type liquid crystal display apparatus having the front light.
The low light utilization efficiency is a more serious problem in the case of a reflection-type liquid crystal display device without using the back light. More specifically, a reflection-type color liquid crystal display device provided with the RGB color filters can ensure a sufficient viewability in extremely bright outdoors. On the other hand, however, it is difficult to ensure the sufficient viewability not only in a dark place but also in an environment of brightness in office or home.
On the other hand, as a color liquid crystal display apparatus for effecting color display without using the color filter, an ECB-type liquid crystal display apparatus, as proposed by U.S. Pat. No. 6,014,195, has been known. The ECB-type liquid crystal display apparatus is generally constituted by a pair of substrates and liquid crystal sandwiched between the substrates, and is roughly classified into those of a transmission-type and a reflection-type.
In the case of the ECB-type liquid crystal display apparatus of the transmission-type, each of the pair of substrates is provided with a polarization plate. On the other hand, in the case of the ECB-type liquid crystal display apparatus of the reflection-type, there are one-polarization plate type display apparatus in which only one of the substrates is provided with a polarization plate and two-polarization plate type display apparatus in which both of the substrates are provided with a polarization plate and a reflection plate is disposed outside each of the polarization plate.
In the case of the ECB-type liquid crystal display apparatus of the transmission-type, linearly polarized light which comes in through one of the polarization plates is changed into elliptically polarized light consisting of respective wavelength light fluxes different in state of polarization by the action of birefringence of liquid crystal layer in a process of transmitting a liquid crystal cell. The elliptically polarized light enters the other polarization plate and the transmitted light having passed through the other polarization plate is colored light consisting of light fluxes of colors corresponding to light intensities of the respective wavelength light fluxes.
In other words, the ECB-type liquid crystal display apparatus is capable of coloring light by utilizing the birefringence action of the liquid crystal layer of the liquid crystal cell and the polarization action of at least one polarization plate without using the color filter.
As described above, the ECB-type liquid crystal display device causes no light absorption by the color filter, so that it is possible to effect bright color display at a high transmittance of light.
In addition, in the ECB-type liquid crystal display apparatus, the birefringence of the liquid crystal layer is changed by an alignment state of liquid crystal molecules depending on a voltage applied between electrodes of both of the substrates of the liquid crystal cell. In correspondence thereto, the state of polarization of the respective wavelength light fluxes entering the other polarization plate is changed. For this reason, by controlling the voltage applied to the liquid crystal cell, it is possible to change the color of the colored light. As a result, it is possible to display a plurality of colors at one (the same) pixel.
In the case where the ECB-type liquid crystal display apparatus of the transmission-type is driven in a crossed-Nicol condition, it is found that the color is changed depending on an amount of retardation, i.e., birefringence. In the case where, e.g., the liquid crystal device uses a liquid crystal material having a negative dielectric anisotropy (−Δε) such that liquid crystal molecules are homeotropically (vertically) aligned to assume black under no voltage application. With an increase in voltage, the color is changed in the order of black-gray-white-yellow-red-violet-blue-yellow-violet-light blue-green.
However, in such a conventional liquid crystal display apparatus which effects display in the above described ECB mode, it is possible to effect arbitrary color display at the same pixel. However, the ECB mode is a mode utilizing coloring by retardation, so that there has arisen such a problem that a display color is changed by a change in retardation with temperature.
Further, when there is an irregularity in temperature in a panel plane, the temperature irregularity is visually identified as an irregularity in display color. It is possible to obviate the temperature irregularity by performing temperature compensation in principle. However, when fine temperature compensation is performed, it leads to an increase in total production cost of the entire display apparatus.
In the ECB mode, a viewing angle characteristic is also limited. Further, in the ECB mode, it is difficult to effect full-color display although multi-color display can be effected.
Further, in the ECB mode, it is clear from its color display principle that the display color is largely changed by a change in cell thickness. Accordingly, in such a process that a uniform cell gap is provided by using a pair of (upper and lower) substrates in combination, the ECB requires change for more strictly than other display modes. As a result, it is considered that this requirement is a large impediment to improvement in production yield.